Since the advent of higher compression automobile and aircraft gasoline engines in the late 30's and 40's, the demand for higher octane gasoline has continually risen. For the past many years, this octane requirement has been supplied by the addition of various organolead compounds, notably tetraethyl lead (TEL), or other similar compounds, to mixtures of various hydrocarbons. However, because of the widespread use of catalytic convertors in removing undesirable components from the exhaust gases of automobiles (which convertors are poisoned by the use of lead in gasoline), other methods of improving motor gasoline octane became more important. One such method of improving the octane of a straight run-gasoline fraction is catalytic reforming.
Reforming may be practiced on naphtha feedstreams which have been desulfurized. Naphthas, "straight run" or otherwise, are generally obtained from simple distillation of a crude oil stream in a "pipe still". Straight run naphtha is typically highly paraffinic but may contain significant amounts of naphthenes and minor amounts of aromatics or olefins. In a typical reforming process, the reactions include dehydrogenation, isomerization, and hydrocracking. The dehydrogenation reactions typically will be the dehydroisomerization of alkylcyclopentanes to aromatics, the dehydrogenation of paraffins to olefins, the dehydrogenation of cyclohexanes and cyclohexenes to aromatics, and the dehydrocyclization of paraffins and olefins to aromatics. The aromatization of the n-paraffins to aromatics is generally considered to be the most important because of the high octane of the resulting aromatic product. The isomerization reactions include isomerization of n-paraffins to isoparaffins, the hydroisomerization of olefins to isoparaffins, and the isomerization of substituted aromatics. The hydrocracking reactions include the hydrocracking of paraffins and hydrodesulfurization if any sulfur compounds remain in the feedstock. On lighter naphtha streams, it is often desirable to avoid hydrocracking because of the low-carbon-number gaseous products which may result.
It is known that several catalysts are capable of reforming petroleum naphthas and hydrocarbons that boil in the gasoline boiling range. Examples of known catalysts useful for reforming include platinum (and optionally rhenium or iridium) on an alumina support, platinum on type X and Y zeolites, platinum on intermediate pore size zeolites as described in U.S. Pat. No. 4,347,394, platinum on cation exchanged type L zeolites, and palladium on mordenite.
As noted above, the typical reforming catalyst is a multi-functional catalyst which contains a metal hydrogenation-dehydrogenation component which is usually dispersed on the surface of a porous inorganic oxide support, notably alumina. Platinum has been widely commercially used in recent years in the production of reforming catalysts, and platinum on alumina catalysts have been commercially employed in refineries for the past few decades. In the last decade, additional metallic components, e.g., iridium, rhenium, tin and the like, have been added to platinum as promoters to further the activity, selectivity, or both, of the basic platinum catalyst. Some catalysts possess superior activity, or selectivity, or both, as contrasted with other catalysts. Platinum-rhenium catalysts, by way of example, possess high selectivity in contrast to platinum catalysts. Selectivity is generally defined as ability of the catalyst to produce yields of C.sub.5 + liquid products with concurrent low production of normally gaseous hydrocarbons (i.e., methane) and coke. Multi-metallic catalysts containing platinum are discussed in length, see Bimetallic Catalyst, J. H. Sinfelt, John Wiley, New York, 1985.
In a typical reforming operation, one or a series of reactors, or a series of reaction zones, are employed. Typically, a series of reactors are employed, e.g., 3 or 4 reaction vessels, which constitute the heart of the reforming unit. Although there are cases where split feed operations are practiced, the typical reaction scheme involves a set of serial feed reactors.
It is known that the amount of coke produced in an operating run increases progressively from the leading reactor to subsequent reactors as a consequence of the different types of reactions that predominate in the several different reactors. The sum total of the reforming reactions occurs as a continuum between the first and last reactor of the series. The reactions which predominate among the several reactors differ principally upon the nature of the feed and at the temperature employed within the individual reactors. In the initial reaction zone, which is maintained at a relatively low temperature, the primary reaction involves dehydrogenation of naphthenes to produce aromatics. The isomerization of naphthenes, particularly C.sub.5 + and C.sub.6 napthenes, also occurs to a considerable extent. Most of the other reforming reactions also occur, but only to a lesser extent. There is relatively little hydrocracking, and very little olefin or paraffin dehydrocyclization occurring in the first reactor. Typically, the temperature within the intermediate reactor zones is maintained at a somewhat higher level than in the first or lead reactor series. Primary reactions in these intermediate reactors involve the isomerization of naphthene and paraffins. Where, for instance, there are two reactors placed between the first and last reactor in series, the principal reaction in these middle two reactors involves isomerization of napthenes, normal paraffins and isoparaffins. Some dehydrogenation of napthenes may, and usually does, occur at least within the second of the four reactors. The amount of hydrocracking increases in the second reactor as does the gross amounts of olefin and paraffin dehydrocyclization.
The third reactor of the series is generally operated at a moderately higher temperature than the second reactor. The naphthene and paraffin isomerization reactions continue as the primary reaction in the reactor, but there is very little naphthene dehydrogenation. There is a further increase in paraffin dehydrocyclization, and more hydrocracking. In the final reaction zone, which is typically operated at the highest temperature of the series, paraffin dehydrocyclization, particularly dehydrocyclization of the short chain or C.sub.6 and C.sub.7 paraffins, is the primary reaction. The isomerization reactions continue and there is often more hydrocracking in this reactor than in any other reactor of the series.
Few processes are, however, capable of allowing adjustment of the aromatics to isoparaffin ratio by mere adjustment of the temperature or by selection of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio of the zeolite used in the catalyst.
The catalysts used in this process are zeolites having the faujasite structure, natural or synthetic, having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of greater than about 6.0 and contain highly dispersed Group VIII noble metal, particularly platinum.
Other dealuminated zeolites are known. Typical processes suitable for dealuminating zeolites include the following:
The U.S. patent to Kerr et al, U.S. Pat. No. 3,442,795, shows dealumination of various zeolites with complexing agents, notably EDTA. PA1 Eberly et al (U.S. Pat. No. 3,591,488) discloses a process for dealuminating zeolites (e.g., faujasites, mordenite, and erionite) using steam. The resulting materials are said (at column 5, lines 17 et seq) to be useful in a wide variety of hydrocarbon conversion reactions. Their use in hydrocracking is highlighted at column 5, lines 43 et seq.
The U.S. patent to Pickert (U.S. Pat. No. 3,640,681) suggests the use of acetylacetonate to dealuminate partially a variety of large pore zeolites such as type X, type Y, type L and mordenite.
Kerr et al (U.S. Pat. No. 4,093,560) teaches a method for dealuminating zeolites using an alkali or ammonium salt, preferably one of EDTA.
Rollman (U.S. Pat. No. 4,431,746) also suggests using a complex containing transition metals of low or zero ion charge to dealuminate zeolite materials.
Breck et al describes methods amounting to Al framework exchange using (NH.sub.4).sub.2 SiF.sub.6 solutions (U.S. Pat. No. 4,503,023).
Dealuminated zeolites (or zeolites otherwise having high SiO.sub.2 /Al.sub.2 O.sub.3 ratios) are used in a wide variety of hydrocarbon conversion operations:
______________________________________ U.S. Patentee Pat. No. Metal Zeolite Process ______________________________________ Benesi 3,475,345 Pt mordenite paraffin isomerization Chen et al 3,673,267 Group mordenite paraffin IB, VIB isomerization VIII Peck et al 3,953,320 noble mordenite cracking metals isomerization Goring et al 3,980,550 noble ZSM-5 etc. hydrodewaxing metals Rollmann 4,148,713 pref. ZSM-5 etc. alkylation, Pt etc. Rosinski et al 4,309,280 Re ZSM-5 etc. cracking Miller 4,330,396 various ZSM-5 etc. "upgrading" ______________________________________
The use of faujasite zeolites (sometimes of high SiO.sub.2 /Al.sub.2 O.sub.3 ratio) to isomerize or aromatize various hydrocarbons has been shown.
U.S. Pat. No. 3,714,029 to Berry shows the use of zinc-substituted type Y zeolite to effectuate hydroisomerization of paraffins.
Buss et al (U.S. Pat. No. 4,435,283) teach the use of, inter alia, ziolites L, X and Y with Group VIII noble metal (particularly platinum) and alkaline earth metals as reforming catalysts.
A British patent (No. 1,506,429) to Best et al teaches the use of a zeolite Y, potentially of an enhanced SiO.sub.2 /Al.sub.2 O.sub.3 ratio in a number of hydrocarbon conversion processes including reforming, isomerization and many others. The maximum disclosed SiO.sub.2 /Al.sub.2 O.sub.3 ratio appears to be 5.4.
None of the cited prior art appears to suggest a process for paraffin isomerization and aromatization with low cracking using a high silica faujasite and containing a highly dispersed Group VIII noble metal, desirably platinum.